Opioid peptides (OP) are coreleased with catecholamines (CA) from nerve terminals in the heart cardiac myocytes also produce and secrete opioids and have opioid peptide receptors (OPR). Thus we proposed that OP also may have a postsynaptic cardiac intracellular signalling role which involves a potent "cross-talk" with the beta-adrenergic receptor (beta-AR) stimulation pathway. The focus of this work was to identify whether OPR stimulation influences the effects of beta-AR stimulation and the nature of the mechanism(s) of interaction. In an intact isolated heart preparation (pretreated with 6-hydroxy-dopamine to deplete CA from peripheral nerve endings) peak systolic pressure was increased to 217% of control by the CA norepinephrine (NE;10-7M), addition of the delta-OPR agonist leucine enkephalin (LE;10-8M) resulted in a marked reduction in developed pressure to 66% of control within 15-25min. The OPR antagonist naloxone (10-8M) added to the LE+NE buffer rapidly reversed the LE effect (<1-2min) to 188% of control systolic pressure. Although 10-8M LE potently inhibited the positive inotropic effect of NE including the stimulated increase in cAMP, alone, at this concentration it had no effect on systolic pressure nor tissue content of cAMP. A non-hydrolyzable analog of cAMP, CPTcAMP, at 3x10-5M increased systolic pressure to 176% of control but LE (10-8M)+CPTcAMP could not counter act the positive inotropic effect. Similarly during perfusion with forskolin, which raised systolic pressure to 200%, 10-8M LE also had no effect. Following pretreatment with pertussis toxin, (which catalyzes the adenine nucleotide ribosylation of Gi/G0-protein alpha subunits and inhibits the response to agonists), LE could no longer inhibit the effects of betaAR stimulation. Similar results were observed in single cardiac myocytes in which cytosolic Ca2+ and Ica could be measured. We conclude that the potent effects of LE are due to a specific "cross-talk" of the LE signalling cascade on the betaAR stimulation pathway and mediated by a pertussis toxin-sensitive G protein involved in the inhibition of adenylyl cyclase. This interaction may thus regulate the magnitude of beta-adrenergic effects on cardiac work and provide protection by preventing metabolic substrate supply/demand imbalance during intense cardiac stress such as exercise or ischemia.